A wireless local area network (LAN) is a network that allows mobile users to access network resources through a wireless radio connection. 802.11b wireless networks are increasingly being adopted as a means of providing low cost wireless infrastructure in business and industry settings. These developments are occurring as 802.11 radios are being integrated as standard components within mobile computing devices such as laptops and PDAs. It should be appreciated that most conventional uses of wireless LANs involve data delivery. These LANs provide simple and low cost wireless connectivity and data delivery. However, their use in low-delay applications such as voice over IP (VoIP) and video over IP (video phones) represents an emerging frontier in wireless network communications.
In contrast to data communications (which are very sensitive to data packet losses but typically not sensitive to delays), voice and video communications can tolerate some losses, but have strict delay requirements. Specifically, if a voice or video packet arrives late, it is rendered useless, which is equivalent to it being lost. Consequently, the quality of the transmissions that involve such effective data packet losses can be severely impacted. Issues such as this must be addressed to provide satisfactory voice and video communications systems on 802.11.
There has been considerable work on 802.11 networking for data delivery, and also some work on low latency communication over 802.11b. Some of this work included the examination of the link-layer behavior for UDP traffic as a function of data packet size and the effect of Bluetooth or microwave oven interferers on 802.11 bandwidth and delay performance. In addition, there have been various approaches suggested for the transmission of video over 802.11.
In one suggested approach, forward error correction (FEC) is used to overcome time-varying wireless losses (for example by adding 50 parity data packets to every 100 data packets) leading to a delay of 100-50 data packets (depending on the loss pattern). This level of data packet delay renders this approach inappropriate for low delay applications. In another approach, error-resilient source coding coupled with path diversity has been examined for multiple description video coding and path diversity over data packet networks, low-latency voice over IP using the distributed infrastructure of a content delivery network (CDN) to achieve path diversity between multiple senders in a CDN and each client, and using path diversity over ad-hoc wireless networks for MD image communication.
FIGS. 1A, 1B and 2 illustrate an aspect of the operation of a conventional hand-off transmission scheme. FIG. 1A shows a series of transmitted data packets d1-d17 transmitted using a conventional handoff technique (such as is illustrated in FIG. 1B) during a data transmission period t0-tx (where d1 is transmitted at time t0 and dx is transmitted at time tx). Conventional handoff techniques attempt to improve data transmission by effecting a change of route in a data transmission path. Transmission over an identified channel (such as d1-d17 over channel AP1 in FIGS. 1A and 1B) is effected until a channel that presents more favorable transmission conditions (such as channel AP2 in FIG. 2 for transmission of packets d18-d34 during period tx+1-tn) is identified. It should be appreciated that handoff techniques feature the effecting of simple one dimensional changes in the dominant route of the transmission path that is used. A drawback of such techniques is that handoffs between access points can be lengthy and can cause interruptions in delay sensitive applications such as VoIP. Another drawback of such techniques is that in many cases the decision rule that is employed to select the handoff path does not yield expected results (e.g., the transmission of a data packet over a path judged to be optimal by a handoff decision rule proves unsatisfactory).
Some cellular systems utilize two access points that each transmit waveforms containing the same data as a means of improving transmission results. These waveforms are transmitted and received simultaneously. The waveforms are combined at the physical layer by the client or hand set. The physical layer for cellular systems is arranged such that only the simultaneous transmission of such waveforms is supported. It should be appreciated that some wireless networks such as 802.11 are not designed to facilitate simultaneous transmissions of this sort. Such transmissions in the 802.11 context could result in transmission failure due to catastrophic data packet collisions.
Conventional wireless transmission schemes such as those discussed above exhibit highly variable delays, substantial data packet losses and significant bandwidth constraints. These transmission schemes can perform satisfactorily when employed in data delivery applications such as web browsing, checking e-mail and performing file down loads. However, because of the delays that they exhibit, such transmission schemes do not perform satisfactorily in voice over IP (VoIP) or video over IP (video phone) applications.